It is known that the flammability of polymers and plastics may be reduced by the addition of fire retarding agents. It is also known that these fire retarding agents may be mixed with thermoplastic polymers in concentrated forms, sometimes called master batches, for mixing with other thermoplastic polymers and plastics to impart fire retardancy.
A large number of substances have shown properties of retarding fire and reducing flammability of thermoplastics, and some are appropriately used as fire retarding agents in thermoplastic polymers and plastics. These fire retarding agents are usually solids exhibiting several of the following properties: high melting or sublimation points, endothermic degradations, resistance to attrition, and intumescence.
These fire retarding agents may be blended directly with thermoplastic polymers in a conventional dry blender such as a V-type blender. The fire retarding agents are usually available as powders, but the thermoplastic polymers are normally available as extrusions or chips. Powders and extrusions are difficult to blend and impossible to keep and use in a homogeneous condition.
The fire retarding agents may be melt blended by mixing in a mass of molten thermoplastic resin, and then discharged to be broken up into appropriately sized solid particles. The normal method for breaking up the particles consists of extruding molten material through a die, and cooling the extruded die strans until they solidify. The solidified strans are then chopped into desired lengths. The method most widely used continuously feeds the fire retarding agent in powder form to a twin screw melt blender along with the thermoplastic polymer. Although the method produces satisfactory fire retardants, it has several severe drawbacks and limitations.
Screw type melt blenders are expensive and the wear on the screws from feeding powders of fire retarding agents is substantial, costly, and requires periodic shutdowns to repair, rework, or replace blender screws and barrels worn by the abrasive fire retardant powders.
In either of the aforementioned methods of preparing fire retardants for thermoplastics, concentrates of fire retarding agent in thermoplastic polymers may be prepared which may be in turn melt blended with other thermoplastic polymers and plastics to impart fire retardancy thereto. However, such a practice is strongly limited by the relatively low concentrations which may be prepared, the lack of homogeneity, the poor mechanical properties of the concentrated fire retardants prepared, and poor or impossible mechanical operations at high fire retarding agent concentrations.
Another problem of the prior art methods of preparing thermoplastic fire retardant particles, is that the thermoplastic polymers used in the methods must be substantially free of water or low boiling solvents. If more than a trace of moisture is added to, or contained by, the thermoplastic polymer in the methods of the prior art, the apparatus must be equipped with special vents or the fire retardant particles produced will contain foam caused by the liquids evaporating in the melt blender.
The use of a screw type melt blender in producing fire retarding thermoplastic resins is demonstrated by Tsutsami in U.S. Pat. No. 4,001,177, where he was able to produce a flame retarding composition containing between 3 and 40 percent melamine and between 0.5 and 20 percent isocyanuric acid. In U.S. Pat. Nos. 4,321,189 and 4,363,890, Oshita and Tsutsami advanced the art further in producing a flame retardant melamine cyanurate in nylon. However, they were limited to a maximum fire retarding agent concentration of about 30 percent, and higher concentrations downgraded the mechanical properties of the nylon.